That's interesting because back in December I opened a ticket for the same problem. After several troubhleshooting attempts, support suggested I RMA the phone, an S300, so I did. Received the new S300, worked fine for about a week or so, now back to locking up and rebooting daily. Support thought maybe something with my internal network. I've checked just about everything imaginable. Interesting point to consider, I have two softphone extensions running on two different computers and haven't experienced any issues with these extensions. I would think if it was something with my network config or my FreePBX config, the softphones would experience issues as well.
Just offering my own experience to let you know you're not alone. Still haven't resolved the issue with the new S300. Considering purchasing an S400 or S500, but reluctant to spend the money because I'm not sure the issue is specific to the S300 or global to all Sangoma phones.
Wish I had a resolution.
Have a great day.