updating firmware on Linksys router

I have Vonage service and am paying about $35 a month for the 15 buck a
month service plus international calls for which they charge me. I want to
change service if they want to overcharge me for this service, especially as
I discovered that Vonage charge me 30 cents a minute for calls to cellular
phones abroad whereas Broadvoice 'only' charge 23 cents.

I just got connected to an English VOIP service and wanted to change to
Broadvoice who will charge a flat $20 for all this service and provide me
with the same Linksys RPC300 router. But THEN I discovered that like the
Vonage service, Broadvoice's routers are supposedly locked into their
service and I can't use the unused second RS232 port for the UK VOIP
provider.

Is there a way of unlocking any of these RPC300 routers by for example
changing or updating the firmware?