I have to dial number twice

[reader]

I have my ATA (NB9W) registered with voxalot.com.au using Pennytel and Sipme routed by Voxalot dial plans.
Since, I guess about 5 days ago, whenever I first dial, the number sounds like it is engaged when I know it isn't, then if I hit redial it rings properly. This is regardless of which provider it is to be routed through.

If I just keep testing the dialing, every second attempt consistently will ring properly with the other attempts giving an engaged tone. When it is going to work the wait for switching takes a bit longer. I'm dialing my PSTN number so I'm sure its not engaged.

If I register my ATA directly to the provider there is no problem.

Although the voxalot aus server worked ok for me earlier, I tried changing back to the US server but that gives an engaged tone all the time for me now.

If I register my ATA directly to the provider there is no problem so for the moment I've left it like that.

Any ideas?

My Voxalot number is 523004.