In addition to all the comments I also suggest reducing the SIP timers and retries under sip-ua. This is to prevent excessive delay is the SIP service goes down, before the Options Ping polling shuts down the dial peer. Default timers/retries would have it trying for something like 30 seconds. I normally use two retries and a reasonably short timer, the exact figure would depend on how quickly your provider normally responds. Remember the timer doubles on each retry, so this example would timeout after 1.75 seconds ..
retry invite 2
timers trying 250