Snom lists a number of supported USB WLAN chipsets and occasionally adds new ones in newer firmwares. I had a BIGtec BIG120, but it died recently (it would overheat after a few days’ use and then fail to see any networks beyond channel 1 until it cooled down). Most of the devices listed by Snom are ancient; my BIG120 is no longer available in the market. Firmware 8.9.3.60 added some new supported WiFi chips, but Snom’s list of supported devices is rather short. There are two additional troubles with USB WiFi adapters: Manufacturers often release new revisions under the same name but with a different chip, so you have to be careful to buy just the right one. And while the phone (or other Linux device) might support that specific chip, its driver needs to actually contain the USB ID of the specific USB adapter. You can find information about revisions and about USB IDs of all kinds of devices at WikiDevi.
Here is a list of all the USB IDs supported by the Snom D715 (and probably all D7X5 and D3X5 phones) that I extracted from firmware 8.9.3.80. I didn’t include the Ralink RT3070/RT2070 that were already supported by the older Snom 7×0 and 8xx series as they are difficult to find nowadays, so unless you have one of the older phones, don’t bother with them.
8192cu.ko 04BB:094C 04BB:0950 04F2:AFF7 04F2:AFF8 04F2:AFF9 04F2:AFFA 04F2:AFFB 04F2:AFFC 050D:1004 050D:1102 050D:2102 050D:2103 0586:341F 06F8:E033 06F8:E035 0789:016D 07AA:0056 07B8:8178 07B8:8189 0846:9021 0846:9041 0846:F001 0B05:17AB 0B05:17BA 0BDA:018A 0BDA:0A8A 0BDA:17C0 0BDA:1E1E 0BDA:2E2E 0BDA:317F 0BDA:5088 0BDA:8170 0BDA:8176 0BDA:8177 0BDA:8178 0BDA:817A 0BDA:817B 0BDA:817C 0BDA:817D 0BDA:817E 0BDA:817F 0BDA:8186 0BDA:818A 0BDA:8191 0BDA:8191 0BDA:8754 0DF6:0052 0DF6:005C 0DF6:0061 0DF6:0070 0E66:0019 0E66:0020 0EB0:9071 103C:1629 1058:0631 13D3:3357 13D3:3358 13D3:3359 2001:3307 2001:3308 2001:3309 2001:330A 2001:330B 2019:1201 2019:4902 2019:AB2A 2019:AB2B 2019:AB2E 2019:ED17 20F4:624D 20F4:648B 2357:0100 4855:0090 4855:0091 4856:0091 7392:7811 7392:7822 CDAB:8010 CDAB:8011 8192eu.ko 0BDA:818B 0BDA:818C 2001:3319 2357:0107 2357:0108 2357:0109 mt7601Usta.ko 148F:6370 148F:7601 148F:7650 rt5572sta.ko 0411:00E8 043E:7A12 043E:7A13 043E:7A22 043E:7A32 043E:7A32 043E:7A42 0471:200F 0471:20DD 0471:2104 0471:2126 0471:2180 0471:2181 0471:2182 04BB:0944 04BB:0945 04BB:0947 04BB:0948 04DA:1800 04DA:1801 04DA:23F6 04E8:2018 050D:8053 050D:805C 050D:815C 050D:815C 057C:8501 0586:3416 0586:341A 0586:341E 0586:343E 0789:0162 0789:0163 0789:0164 0789:0166 07AA:002F 07AA:003C 07AA:003F 07B8:2770 07B8:2870 07B8:3070 07B8:3071 07B8:3072 07D1:3C09 07D1:3C0A 07D1:3C0D 07D1:3C0E 07D1:3C0F 07D1:3C11 07D1:3C16 07D1:3C17 07FA:7712 083A:6618 083A:7511 083A:7512 083A:7522 083A:8522 083A:A618 083A:A701 083A:A702 083A:A703 083A:B511 083A:B511 083A:B522 0846:9012 0930:0A07 0B05:1731 0B05:1732 0B05:1742 0B05:1784 0CDE:0022 0CDE:0025 0DB0:3820 0DB0:3821 0DB0:3822 0DB0:3870 0DB0:3871 0DB0:6899 0DB0:6899 0DB0:821A 0DB0:822A 0DB0:822B 0DB0:822C 0DB0:870A 0DB0:871A 0DB0:871B 0DB0:871C 0DB0:899A 0DF6:0017 0DF6:002B 0DF6:002C 0DF6:002D 0DF6:0039 0DF6:003E 0DF6:003F 0DF6:0041 0DF6:0042 0DF6:0042 0DF6:0047 0DF6:0048 0DF6:0050 0DF6:005F 0DF6:0065 0DF6:0066 0DF6:0067 0DF6:0068 0E66:0001 0E66:0003 0E66:0021 100D:9031 1044:800B 1044:800D 129B:1828 13B1:002F 13D3:3247 13D3:3273 13D3:3305 13D3:3307 13D3:3321 13D3:3329 13D3:3329 13D3:3365 1482:3C09 148F:2770 148F:2870 148F:3070 148F:3071 148F:3072 148F:3370 148F:3572 148F:3573 148F:5370 148F:5372 148F:5572 14B2:3C06 14B2:3C07 14B2:3C09 14B2:3C12 14B2:3C23 14B2:3C25 14B2:3C27 14B2:3C28 157E:300E 15A9:0006 15C5:0008 167B:4001 1690:0740 1690:0740 1690:0744 1690:0761 1690:0764 1737:0070 1737:0071 1737:0078 1737:0079 1740:9701 1740:9702 1740:9703 1740:9705 1740:9706 1740:9707 1740:9708 1740:9709 1740:9801 177F:0302 1875:7733 18C5:0012 1A32:0304 1D4D:000C 1D4D:000E 1D4D:0011 1EDA:2012 1EDA:2012 1EDA:2210 1EDA:2310 2001:3C15 2001:3C19 2001:3C1A 2001:3C1B 2001:3C1C 2001:3C1D 2019:5201 2019:AB25 2019:ED06 2019:ED19 203D:1480 203D:14A9 20B8:8888 5A57:0280 5A57:0282 5A57:0282 5A57:0283 5A57:0284 5A57:5257 7392:4085 7392:7711 7392:7717 7392:7718 7392:7733
My choice fell on the ASUS USB-N10 Nano, which is on the list as 0B05:17BA. It is available for 10 Euro, is so small that it fits into the side USB port without protruding from behind the phone, works stable and does not produce a lot of heat.