SIP Session Timer on Remote Phones

SNOM 300 phones
I had a terrible time getting my remote phone at home working until someone suggested I change the SIP session timer(s) from the default of 3600 to 60. Works fine ever since.

Now, I am moving my business so for a week or so, ALL my phones will be remote. Doesn't seem so easy. Phone seem to keep losing connection -- one minute they;re working fine then some of the phones stop working. SIP session times doesn't seem to be working as the fix.
When I reregister the phone from the browser interface, seems to be fine for a while.

Basically, I get dialtone, but nothing happens when I dial.
Proposed Expiry and Subscription Expiry, NTP refresh are still set at 3600

There's something I don't fundamentally understand here --
do the phones re register after a certain period?
if I set the re register to a short period, does this cause a problem?
which setting controls re register time?

Thanks! Hope you can help.