For the last three months approx. one of my dogs howls every time the phone rings, and when I pick up the phone for a minute or two. How can I stop this unwanted behaviour?  

I say no firmly but it doesn't make any difference even though he is looking at me.

Change the ring tone.

Since my eldest bitch passed my other two have done this every time the phone rings. One starts and that sets the other off, they also do it when the  answer machine comes on. I've tried "no" , diversion tactics etc and so far nothing has helped so am really interested in others suggestions.

Maybe try distracting her with a treat or a chew.

ok I will try the suggestions.

I had something similar with my sheltie.  Phone ringing and oven timer.  It took a long time and a lot of patience.  I never gave her a treat as she associated it with a reward.  I ignored the phone and her and busied myself with other chores.  For example when in the kitchen I started washing up.  In the living room I continued knitting or reading.  As for the oven timer I bought a hand held one that had a different sound and carried it with me so the minute it went off I switched it off.  When I went out I switched the phone to silent so she didn't hear it and it went straight to answer phone.
Not a quick solution but if you started now she should be alright by Xmas. (lol)

I agree, change the ring tone, but also start to teach howling on cue and then quiet on cue. You can use the old ring tone to trigger and teach howling on cue.

Once you have taught both howl cue and quite cue you can use a cued, quick howl as a reward for being quiet for extended periods. Sounds like your dog enjoys a howl, some do.

My girl used to howl at OH mobile phone. Then the house phone. It never made any difference changing the ringtone, she still did it to the new one. I always rather liked it, and people phoning me knew they had to wait a few seconds for the howling to subside. So, I can't help on how to stop it as I never wanted to and never tried, but do know that ringtone made no difference whatsoever to her.

Slightly off subject but I had to laugh yesterday. I stopped for petrol and a fire engine drove past with  it's sirens going, a man was walking along with his two Malamutes and they both in unison put their heads back and started to howl in reply to the sirens.

My 'Imitative Howling' post described the same. So weird. Fern gives them the A and off they go. I yell ' Have I got a pack of wolves?' and they shut up.