Yes, you can twin a VOIP / desk phone handset with your SIM.
The important thing to remember is that if you twin your service, only one call can be made/received at any one time. So for example if you are on a call on your SIM and another caller tries to ring you, they will not be able to get through to either the SIM or VOIP handset as the line will be engaged and the call will follow your busy routing (the default would be for the second call to go to voicemail).
If you would like to be able to make/receive multiple calls at any one time we can set the VOIP handset up with its own separate line to achieve this for you, the VOIP line would just need its own separate Base plan.
Compatible VOIP handsets are available to purchase through ONSIM.
If you would like to proceed with setting up the twinned service and purchasing a VOIP handset please email email@example.com
Our support team may ask you to perform a speed test using the internet connection you intend to use the VOIP handset on to ensure that the broadband speeds are sufficient to provide you with good voice quality on the VOIP handset. You can perform this speed test here and email a copy of the results to firstname.lastname@example.org to check this with the team.