MONTREAL/CHICAGO (Reuters) - U.S. and Canadian carriers wrestled with customer calls and flight cancellations on Thursday after the two countries followed other nations in grounding Boeing’s 737 MAX planes because of safety concerns.

WestJet airline signage is pictured at Vancouver's international airport in Richmond, British Columbia, Canada, February 5, 2019. REUTERS/Ben Nelms

Dallas-based Southwest Airlines Co, the largest operator of the 737 MAX in the world with 34 jets, said it was experiencing unusually high call volumes and had canceled 39 MAX flights.

The low-cost carrier, a launch customer of the MAX 8 in 2017, said it was servicing those flights with available 737-700s and 737-800s.

Canada and the United States announced on Wednesday they would ground the MAX planes, citing new satellite data and evidence from the scene of an Ethiopian Airlines plane crash on Sunday that killed 157 people.

American Airlines Group Inc, with 24 MAX 8s, planned to cancel around 85 flights per day as a result of the ban, while servicing some of those flights with other aircraft such as the 737-800.

Air Canada, the country’s largest carrier, which operated 24 MAX jets had a message on its customer service line saying that because of “unforeseen circumstances,” call volumes temporarily exceeded its capacity to answer or place calls on hold.

Air Canada could not be reached for comment on Thursday. In a statement on Wednesday, the Montreal-based carrier said it operated 75 737MAX flights daily out of a total schedule of approximately 1,600 daily flights system-wide, representing less than six percent of the network’s total flying.

WestJet Airlines, Canada’s second-largest carrier which operated 13 MAX jets, said it canceled 11 flights on Thursday impacting 1,200 passengers, but would re-book more than three-quarters of them by the end of the day.

According to estimates from the IBA Group, the direct costs to airlines for suspending the MAX would be around $150,000 per day per leased aircraft grounded. Boeing 737 MAX 8 and 9 planes will remain grounded for “weeks” at a minimum until a software upgrade could be tested and installed in all of the planes, U.S. lawmakers said on Thursday.

Southwest and American each said on Thursday they were flying empty MAX aircraft to be parked at different maintenance locations during the ban.