Production version of Asterisk?

it depends on your needs, what type of pstn connection you are utilizing, if its MTE or STE.. theres a lot of things to consider.

I ran into one carrier that on outbound calls dtmfmode=rfc2833 didnt work yet inbound calls worked fine. Changing the outbound calls to use dtmfmode=info was sufficient to make the calls work at the expense of some scalability down the road.

I have no first hand knowlege of the 1.4 branch beyond 1.4.30 (which was broken) and reverted all the way back to 1.4.26.3 to fix the dtmf issues it was having. I've been following the 'if it aint broke don't fix it' philosophy and never needed to to to 1.4.35 so I cannot help you on that version. I would say it depends on your carrier. I was having issues passing DTMF over to a PRI on the 1.4.27-1.4.30 versions but some on sip trunks experienced no issues.

call parking in 1.6.2.x works better than having to depend on a buggy app_valetparking.. but until we stop supporting the 1.4 LTS in favor of the 1.8 LTS version, setting it up is going to have to be a rather manual process. the 1.6 branches changed too much, too wildly, and left too many things broken to waste our time trying to support. Our focus going forward is 1.8 and for the most part 1.6.2 will benefit from this.

Ok nice to hear, i guess we stay at 1.4.26.1 then.

We do plan to setup a second carrier så we can let our customer choose between the carrier and have the best mobile coverage from the both carriers and still call the cellphone from the company for free.

Are there any date out for a Thirdlane MTE release of 1.8?

there will be a beta ISO eventually released as 1.8 itself is still BETA. I suspect pbxmanager will call its 1.8 support BETA at least for the first few months after 1.8.0 releases, based on historical behavior of stability.